Crystal frequency resonance monitoring is commonly used to control deposition rates. However, a cable resonance effect occurs in long coaxial cables in quartz crystal deposition controllers.
A Crystal Interface Unit (XIU) is a crystal resonance monitor based on phase locked loop. XIUs house some parts including the phase sensitive detector portion of the measurement circuit only a short cable length away from a quartz crystal sensor mounted inside a thin film deposition chamber. The XIU is then connected to the rest of the measurement circuit (called a measurement card seated inside a rate control and data processing unit) via a multi-conductor control cable whose length can vary up to 30 m. The design of an XIU includes a capacitance bridge to compensate for the phase shift due to the capacitance of the sensor and the reactive conductance of the cable connecting the XIU to a quartz crystal sensor.
The present configuration provides many benefits including the ability to measure the activity (life) of a monitor crystal that is being coated with different materials. However, such benefits are only available to short cable lengths (up to 4.5 m for most crystals with a starting fundamental frequency of ˜6 MHz) and the allowed length further reduces for crystals of higher fundamental frequency. In addition, the maximum length of the sensor cable for an existing XIU also depends on parameters such as cable type, crystal size and geometry, sensor head design, etc.
As the XIU-sensor cable length increases, many of the merits expected of the existing design are lost, primarily due to reflections in coaxial cables, eventually leading to detection failure. Suppressing reflections by impedance matching is a common practice for fixed impedances. The impedance of a quartz crystal, however, varies across a wide range during material deposition. Proper termination in this case (broad frequency band and a wide impedance band) requires multiple matching nodes and a switch circuitry to select the proper matching nodes for crystal impedance falling either side of the specific impedance of the coaxial cable used. Besides the aforementioned issues, long cable lengths also cause increased frequency pulling of crystal resonance and invalid cable compensation track for current XIUs during crystal coating.
Prior schemes experience a reduction in their ability to locate resonance when the electrical length of a XIU-sensor cable (which is a function of both the dielectric constant and the physical length) exceeds a quarter wavelength of the excitation signal. Moreover, a combination of two or more XIUs is required to cover a contiguous length span of up to ˜4 m for an RG58 type coaxial cable.
Superconducting cavity stabilized microwave oscillator circuits for thickness rate monitoring of thin film deposition have been demonstrated. See, e.g., the superconducting cavity stabilized microwave oscillator circuit proposed by S. R. Stein and J. P. Turneaure (published in IEEE proceedings, vol. 63, issue 8-1975). FM signals have been converted to AM signals using a cavity.
In some previous devices, the XIU reduces the cable length from the crystal to the controller. The measurement circuit components are in XIU. The maximum cable length is, for example, 4.5 m from crystal to XIU. Beyond that, reflection and phase reversal can occur. However, single flat-panel deposition chambers for Gen 4 and larger glass are too large, 4.5 m is not enough. Longer cable lengths exhibit reflections and standing waves at ¼ λ. ½ λ has the same impedance as 0 m—impedance is periodic with period ½ λ and reverses every ¼ λ.